This invention relates to a hydraulic speed change gear of the type which includes a fixed displacement hydraulic pump adapted to be driven by an input shaft and a variable displacement hydraulic motor having an output shaft, the motor being connected with the pump by means of a closed hydraulic circuit such that the speed change ratio between the output shaft and the input shaft is controllably varied steplessly by regulating the capacity of the motor.
When a hydraulic speed change gear of the kind as referred to above is used on an automotive vehicle, the closed hydraulic circuit may be utilized as an oil pressure source for various hydraulically operated equipment, such as a power steering system, a brake servo motor, and the like for the purpose of eliminating an expensive hydraulic pump for the exclusive use of the hydraulic equipment. In such case, however, oil pressure in the closed hydraulic circuit is generally subjected to variations in the loading conditions of the speed change gear, so that it becomes necessary for the proper operation of the hydraulic equipment to correct the pressure variations in the closed hydraulic circuit so as to maintain the circuit pressure at any time at a practically constant level.